A dual layer DVD burner is an optical disc player and recorder that can
utilize dual layer DVDs for both playback and burning. Most modern DVD
burners incorporate this function, including those built into computers,
and all models are additionally compatible with standard, single-layer
media.

The advantage of dual layer media is that it holds twice the data of its
single-layer counterpart. A standard single-sided, single-layer DVD
(DVD5) has a capacity of about 4.7 gigabytes (GB), while a single-sided,
dual layer DVD (DVD9) can hold about 8.5 GB. The dual layer disc is
convenient for high-definition movies that can fit on a single disc
without having to reduce quality. Also, both physical layers of data
reside on the same side of the disc so content can play straight through
without need of removing the disc to flip it over.

A commercial pre-recorded optical disc, whether a CD or DVD, has an
internal layer that is stamped with bumps and lands to indicate ones and
zeros for the accurate payback of digital content. The bumps and lands
reside on a spiral track, which a precise laser follows, reflecting off
the deviations. When the laser hits a land, the light bounces back to a
sensor, indicating a one. When the laser hits a bump, the reflection
bounces away from the sensor, indicating a zero.

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When a dual layer DVD burner reads a dual layer disc, the read-laser
readjusts its focus past the semi-transparent first layer of physical
data, on to an additional layer of data. This refocusing of the
read-laser can take a second or two, causing a momentary pause in
playback.

While pre-recorded DVDs and CDs have stamped lands and bumps to suffice
for ones and zeros, writable media used for the home enthusiast utilizes
another method to record ones and zeros. Rather than lands and bumps, a
layer of photosensitive dye is used.

When a dual layer DVD burner is recording data, it uses a write-laser
more powerful than the read-laser. The write-laser heats the dye at
precise points, causing it to become opaque. Where the dye is opaque,
this will be interpreted as a zero. Where the dye remains translucent
allowing the laser to pass through to a semi-transparent, reflective
metal layer, a one. When the first track of data is recorded, the
write-laser re-focuses its beam to a deeper, second layer of dye backed
by a second layer of reflective metal, continuing the burn process.

A dual layer DVD burner can also burn to rewritable media. Rewritable
optical discs use phase-change compounds that can be wiped and rewritten
repeatedly. When a phase-change compound is heated briefly to the
melting point, it de-crystalizes, becoming fluid and opaque, remaining in
this state even after it cools. This serves as a digital zero. If
reheated and held at another temperature (that is interestingly enough
cooler than the melting point), the compound re-crystallizes, becoming
reflective again. In this state it serves up a digital one. A dual layer
DVD burner can use rewritable media to record TV shows or movies for
later viewing, in the same way video cassette recorders were previously
used, recycling a single tape.

Dual layer burners are reasonably inexpensive and are available
everywhere home entertainment equipment and computers are sold. Check
unit specs to verify desired input and output ports, and whether or not
cables are included.

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